Inkjet printers are efficient, quiet and produce high quality print images in a relatively inexpensive manner when operated in low speed printing modes. Such quality is achieved by sweeping a large number of inkjet nozzles over a print medium and ejecting droplets of ink onto the medium in one or more matrix arrays of minute ink drop patterns. Such arrays are known as swaths and the individual ink droplets are defined as pixels. The quality of the print image is then determined by assuring that each ink droplet has a precise volume of ink that is applied to a specific location on the print medium without smearing.
While such low speed inkjet printers have been satisfactory for many applications, there has been a constant demand for higher speed printers that produce high quality full color images. Meeting the demand for higher throughput while producing high quality, high density images, however, has not been achieved easily. In this regard, in order to produce full vibrant colors on a print medium, large volumes of ink must be deposited in concentrated areas on the medium. Such deposits produce vibrant colors but also cause the print medium to buckle and curl, which in turn, greatly effects throughput and print quality as will be explained.
Buckling and curling are technical terms that describe the reaction of an absorbent material, such as bond paper, when a large volume of liquid is deposited in a concentrated area. Buckling which is a problem referred to as cockling, is the expansion of a paper surface upwardly as it absorbs the liquid solvent component of the ink, which is typically water. Curling, on the other hand, is the twisting of the plane of the paper as a result of one side of the paper being saturated with ink while the other side of the paper remains dry.
The effects of cockling and curling are significant. In this regard, in order for an ink droplet to be accurately placed at a specific location on the print medium, the outlet of the inkjet nozzle must be disposed in close proximity to the paper surface. Placement of the nozzle relative to the paper surface however, must be sufficiently spaced to ensure that buckling will not result in the paper surface making contact with the nozzle surface.
Spacing the nozzle too far from the paper surface however, has a detrimental effect. More specifically, although an inkjet process is extremely quiet, it is nevertheless a very violent process. In this regard, each nozzle in the inkjet print head has an inner chamber for receiving a precise volume of ink. The ink enters the chamber through an inlet under capillary action and is ejected from a nozzle outlet with an explosive force as the ink and its constituent solvent are heated rapidly by the application of electrical current to a firing resistor disposed within the chamber. The rapid evacuating of the colorant within the chamber has two effects. First, the ink exiting the chamber expands outwardly to form large and small puddles of ink on the receiving paper which result in fuzzy pixel edges if the nozzle is spaced too far from the paper surface. Second, the ink entering the chamber rushes in against the back fire of the evacuating ink to create a turbulent inflow causing the incoming ink to rise and fall within the chamber as it dissipates its kinetic energy. This firing process is then repeated at a very rapid rate or frequency in order to deposit the large volumes of ink in concentrated areas on the paper. Should the frequency of firing be too rapid there is an immediate image degradation effect as either ink pen starvation or non precise volumes of ink result. Moreover, puddles of ink may accumulate on the nozzle plate which in turn may cause undesired and unwanted droplet trajectory errors.
Several attempts have been made to solve the problems associated with cockling and curling. For example, one solution was to heat the print medium by flowing heated air over the wet ink surface of the medium. Another solution was to heat the print medium while the ink is being ejected onto the medium surface. Other solutions included multi-pass printing and delayed printing to provide greater periods of time for the deposited ink to dry without smearing. While many of these solutions have enjoyed a certain degree of success, with the continuing demand for higher throughput the prior art has not been entirely satisfactory.
One attempt at providing a satisfactory solution for printing high quality graphic images at a high throughput rate is disclosed in the Arbeiter et al. U.S. Pat. No. 5,608,439. The Arbeiter patent discloses a densitometer for adaptive control of ink drying time where a printer controller and an associated algorithm establishes a variable delay time between sweeps. In this regard, the algorithm determines the maximum density of ink to be deposited in a given swath to control the amount of delay time between sweeps. In this manner rather than having a fixed delay time between individual sweeps, a variable delay time is implemented. This technique improves print quality at the expense of throughput and requires large amounts of processor time. Moreover, the Arbeiter et al. patent does not address the problems associated with ink pen starvation.
While the utilization of a variable sweep delay time has been successful in many applications, it would be highly desirable to have a new and improved apparatus and method for improving full color print quality images having densely inked areas in a high speed single pass inkjet printer without inhibiting carriage movement between swaths while simultaneously substantially reducing ink pen starvation, droplet trajectory errors, and fuzzy text edges when printing in a graphic image mode.